The present invention relates to data storage, and more particularly, to improving the error rate performance of headerized sub data sets protected by unequal error protection.
Data sets written on tape include subunits referred to as sub data sets. In 16-channel linear tape open generation 6 (LTO 6), there are thirty-two sub data sets within each three megabyte (MB) data set, whereas in a 32-channel enterprise drive there are sixty-four sub data sets within each six MB data set. All rows of a sub data set, which are also known as codeword interleaves (CWIs) which include four byte-interleaved C1 codewords (row codewords) in LTO 6, are written along different tape tracks as far apart from each other as possible. This minimizes the number of erased bytes in a C2 codeword (column codeword) that occur as a result of a spatial burst error on the tape. In other words, rows of a sub data set are distributed within a written data set in an approximately equidistant manner providing deep spatial interleaving among rows of a sub data set. Furthermore, if all bytes written in a circle having a diameter of about 1 mm cannot be read, at most one erased byte would be caused within a C2 codeword.
A 12-byte header is usually appended to each row of a sub data set giving rise to a headerized sub data set structure. In enterprise drives, headerized sub data sets provide header error protection by embedding header fragments into row codewords of a sub data set making sub data sets robust to header errors. In this case, data is protected by two-level error correction code (ECC) as in the case of product codes, whereas headers are protected by one-level ECC (typically just C1 coding).
Headerized sub data sets with unequal error protection are decoded only by C1 decoding followed by C2 decoding, i.e., no iteration is performed on the headerized sub data sets. This decoding can lead to higher error rates than are necessary, and sometimes acceptable, due to the single C1 decoding followed by C2 decoding. Accordingly, it would be beneficial to have a decoding scheme which improves the error rate performance (reduces the error rate) of decoded headerized sub data sets with unequal error protection.